How do you create an aura, or halo effect, like what was possible with the internal Blender Render Engine, in 2.80
A similar question was asked a little while ago. The following answer should provide you with the information to render halos in Cycles: (see 'Point Clouds').
https://blender.stackexchange.com/a/45487/23799

Since the linked answer is over a year old, I went ahead and verified it's content for Blender 2.80 and expanded upon the explanation on how to set this up.
For further reference on this method, see the aforementioned post.



As an example, I've added a mesh that will represent the location of the particles in the particle system. Add a particle system to the mesh and leave the settings as is.


Add a mesh that completely encompasses the particles in the particle system. This will be your volume shader. Create a material for this mesh and open the Shader Editor.
(I have made the cube semi-transparent so you can see what's going on inside).


In the Shader Editor, detach the Principled BSDF Shader node(or delete it) and add a Point Density shader (Add > Texture > Point Density) and an Emission shader (Add > Shader > Emission).
Connect the Density output of the Point Density node to the Strength input of the Emission shader.
Then connect the output of the Emission shader to the Volume input of the Material Output node.

Set the nodes up as follows; In  the Point Density node, select the object to which you added the particle system(in this case, Suzanne) and select its(her?) particle system (called 'ParticleSettings' here) in the Particle System input below. Leave the other settings as is.


Now, if you render the image using (Viewport) Render, you'll see the particles glow up. You can change the size of the glow by changing the 'Radius' setting in the Point Density node.